While working on a Sublime Text plugin, I needed to capture logging output in a variable so I could display it easily. For once, Stack Overflow1 didn't have an immediate solution. After a couple hours of hacking around, I came up with the following which I hope saves others the hassle of having to build it themselves.

import logging
import io

### Create the logger
logger = logging.getLogger('basic_logger')
logger.setLevel(logging.DEBUG)

### Setup the console handler with a StringIO object
log_capture_string = io.StringIO()
ch = logging.StreamHandler(log_capture_string)
ch.setLevel(logging.DEBUG)

### Optionally add a formatter
formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
ch.setFormatter(formatter)

### Add the console handler to the logger
logger.addHandler(ch)


### Send log messages. 
logger.debug('debug message')
logger.info('info message')
logger.warn('warn message')
logger.error('error message')
logger.critical('critical message')


### Pull the contents back into a string and close the stream
log_contents = log_capture_string.getvalue()
log_capture_string.close()

### Output as lower case to prove it worked. 
print(log_contents.lower())

Note that once you call '.close()', it's possible to run into problems if the same running process tries to call the function again.

This approach will work with external files too. If you assign the same logger (in this case 'basic_logger') it will output properly. For example, if you have an module in an external file called 'external_logger_test.py' with the content:

import logging

def log_test():

	logger = logging.getLogger('basic_logger')

	logger.warning('Log note from external file')

All you have to do is import it as usual in your main file (e.g. import external_logger_test) and then call the function:

external_logger_test.log_test()

Of course, you have to do this before you call the log_contents = log_capture_string.getvalue(). That's where the log data stops flowing.


Links and Notes

  1. Stack Overflow - Friend to developers around the world. Far and away the most useful development research site there is.